U.S. patent number 8,007,118 [Application Number 11/843,178] was granted by the patent office on 2011-08-30 for direct-lit backlight with angle-dependent birefringent diffuser.
This patent grant is currently assigned to 3M Innovative Properties Company. Invention is credited to Mark B. O'Neill, Carl A. Stover, Michael F. Weber.
United States Patent |
8,007,118 |
O'Neill , et al. |
August 30, 2011 |
**Please see images for:
( Certificate of Correction ) ** |
Direct-lit backlight with angle-dependent birefringent diffuser
Abstract
A direct-lit backlight includes a diffuse reflector disposed in
front of a light source. The diffuse reflector has a continuous
phase material and a disperse phase material, at least one of which
is birefringent, and the disperse phase material is formed into
particles having a transverse dimension less than 1 micrometer for
efficient visible light scattering. The continuous and disperse
phases have refractive indices tailored so the diffuse reflector
transmits light from the light source substantially more strongly
for obliquely incident light than for normally incident light to
provide bulb-hiding characteristics.
Inventors: |
O'Neill; Mark B. (Stillwater,
MN), Stover; Carl A. (St. Paul, MN), Weber; Michael
F. (Shoreview, MN) |
Assignee: |
3M Innovative Properties
Company (St. Paul, MN)
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Family
ID: |
39136274 |
Appl.
No.: |
11/843,178 |
Filed: |
August 22, 2007 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20080055881 A1 |
Mar 6, 2008 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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60824196 |
Aug 31, 2006 |
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Current U.S.
Class: |
362/19;
359/489.06; 362/331; 359/485.02; 362/97.2; 362/97.3 |
Current CPC
Class: |
G02B
5/3008 (20130101); G02B 5/0257 (20130101); G02F
1/133606 (20130101); G02B 5/0278 (20130101); G02B
5/0242 (20130101); G02F 1/133607 (20210101); G02F
2202/40 (20130101) |
Current International
Class: |
G02B
5/30 (20060101); F21V 9/14 (20060101) |
Field of
Search: |
;362/19,629,346,97.1-97.3,331,332 ;349/112,61-65 ;428/1.1
;359/488,487.05,487.01,489.06,485.02 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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WO 97/32223 |
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Sep 1997 |
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WO |
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WO 97/32226 |
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Sep 1997 |
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WO |
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WO 01/90637 |
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Nov 2001 |
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WO |
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Primary Examiner: Sember; Thomas M
Attorney, Agent or Firm: Moshrefzaden; Robert S.
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATION
This application claims the benefit of U.S. Provisional Patent
Application No. 60/824,196, filed Aug. 31, 2006, the disclosure of
which is incorporated by reference herein in its entirety.
Claims
The invention claimed is:
1. A direct-lit backlight, comprising: a light source; and a
diffuse reflector disposed in front of the light source, and having
a continuous phase material and a disperse phase material, at least
one of which is birefringent, and whose refractive indices are
tailored to provide the diffuse reflector with a transmissivity
that increases substantially with increasing incidence angle from
normal incidence.
2. The backlight of claim 1, wherein the disperse phase material is
in the form of particles whose average transverse dimension is less
than 1 micrometer.
3. The backlight of claim 1, wherein the diffuse reflector is
spatially uniform.
4. The backlight of claim 1, wherein the transmissivity of the
diffuse reflector differs for obliquely incident light relative to
normally incident light by an amount .DELTA.T, and .DELTA.T exceeds
the combined reflectivity at normal incidence of opposed major
surfaces of the diffuse reflector.
5. The backlight of claim 4, wherein the obliquely incident light
is p-polarized and is incident at a 60 degree angle in air.
6. The backlight of claim 1, wherein the transmissivity of the
diffuse reflector differs for obliquely incident light relative to
normally incident light by an amount .DELTA.T of at least 10%, and
wherein the obliquely incident light is incident at a 60 degree
angle in air.
7. The backlight of claim 6, wherein .DELTA.T is at least 15%.
8. The backlight of claim 1, wherein the continuous and disperse
phases exhibit a relative refractive index mismatch .DELTA.nx along
an in-plane x-direction and a relative refractive index mismatch
.DELTA.nz along an out-of-plane z-direction normal to the
x-direction, .DELTA.nz being substantially smaller in magnitude
than .DELTA.nx.
9. The backlight of claim 1, wherein the disperse phase material is
substantially isotropic and the continuous phase material is
birefringent.
10. The backlight of claim 1, wherein the diffuse reflector is a
diffuse reflective polarizer.
11. The backlight of claim 10, further comprising a second
reflective polarizer disposed in front of the light source.
12. The backlight of claim 11, wherein the second reflective
polarizer is disposed in front of the diffuse reflective polarizer,
and a block axis of the second reflective polarizer is aligned with
a pass axis of the diffuse reflective polarizer.
13. The backlight of claim 1, wherein the diffuse reflector is a
diffuse reflective mirror.
14. The backlight of claim 1, wherein the diffuse reflector has
opposed major surfaces, the backlight further comprising a
structured surface material coupled to at least one major surface
to support super-critical light propagation in the diffuse
reflector.
15. The backlight of claim 1, further including: a back reflector
disposed behind the light source.
16. The backlight of claim 1, further comprising: a secondary
diffuser disposed in front of the diffuse reflector.
17. The backlight of claim 1, wherein the light source is selected
from one or more fluorescent lamps and one or more light emitting
diodes (LEDs).
18. The backlight of claim 1, wherein the diffuse reflector is
spaced apart from the light source by a gap.
19. A combination of optical films suitable for use in a direct-lit
backlight, the combination comprising: a diffuse reflective
polarizer having a continuous phase material and a disperse phase
material, at least one of which is birefringent, and whose
refractive indices are tailored to provide the diffuse reflective
polarizer with a transmissivity along a block axis thereof that
increases substantially with increasing incidence angle from normal
incidence; and a second polarizer having a block axis and a pass
axis, the second polarizer being oriented such that the pass axis
of the second polarizer is aligned with the block axis of the
diffuse reflective polarizer.
20. The combination of claim 19, wherein the diffuse reflective
polarizer and the second polarizer are substantially co-extensive
in length and width.
21. The combination of claim 19, wherein the diffuse reflective
polarizer and the second polarizer are laminated together.
Description
FIELD OF THE INVENTION
The present invention relates to extended area light sources,
particularly those suitable for use as backlights for liquid
crystal (LC) panels, signs, and the like.
BACKGROUND
Recent years have seen tremendous growth in the number and variety
of display devices available to the public. Computers (whether
desktop, laptop, or notebook), personal digital assistants (PDAs),
mobile phones, MP3 music players, and thin LCD TVs are but a few
examples. Although some of these devices can use ordinary ambient
light to view the display, most include a backlight to make the
display visible.
Many such backlights fall into the categories of "edge-lit" or
"direct-lit". These categories differ in the placement of the light
sources relative to the output area or face of the backlight, where
the output face defines the viewable area of the display device. In
edge-lit backlights, a light source is disposed along an outer
border of the backlight construction, outside the area or zone
corresponding to the output face. The light source typically emits
light into a light guide, which has length and width dimensions on
the order of the output face and from which light is extracted to
illuminate the output face. In direct-lit backlights, an array of
light sources is disposed directly behind the output face, and a
diffuser is placed in front of the light sources to provide a more
uniform light output. Some direct-lit backlights also incorporate
an edge-mounted light, and are thus capable of both direct-lit and
edge-lit operation.
One example of a direct-lit display is shown in FIG. 1, which
illustrates in a schematic sectional exploded view selected optical
components of a liquid crystal display television (LCD TV) 10,
commercially available from Samsung Corp. as model LTN226W. This
figure is not to scale, and some of the features are exaggerated
for ease of illustration. The LCD TV 10 has a 22-inch screen size
and a 16:9 aspect ratio, meaning the screen is about 10.8 inches
(275 mm) in height and 19.2 inches (485 mm) in width. In this
regard, height, width, and depth correspond to the Cartesian x-,
y-, and z-axes respectively shown in the figure. The LCD TV 10 has
an LCD panel 12 viewable by the observer 13, and a collection of
components forming a backlight 14 behind the LCD panel 12.
In the backlight 14, light is generated by an array of eight CCFL
sources 16, each of which spans the width of the TV screen and
which have a uniform center-to-center spacing S of about 33.8 mm.
The sources have a circular cross-sectional shape about 3 mm in
diameter (diameter=2 R). They are disposed between a patterned
diffuser plate 18 and a back reflector 20. The diffuser plate 18
transmits light but with a substantial amount of scattering to
provide light diffusing characteristics. The diffuser plate also
has on its back surface a printed dot pattern, which pattern is in
registration with the light sources 16. That is, the printed dot
pattern is more dense at locations directly above the light sources
16 and less dense at locations between neighboring light sources.
Thus, the diffuser plate 18 is highly non-uniform over its useful
area. The back reflector 20 is an opaque white film or paint coated
onto a stiff metal substrate. The setback distance D from the
diffuser plate 18 to the sources 16, measured from the back surface
of the diffuser plate to the center of the sources, is about 11.1
mm, and the distance T from the sources 16 to the back reflector 20
(measured from the center of the sources to the front surface of
the back reflector) is about 5.6 mm. The overall gap dimension G
from the diffuser plate 18 to the back reflector, which is measured
from the back surface of the diffuser plate to the front surface of
the back reflector, and which satisfies G=D+T, is about 16.7 mm.
Atop the diffuser plate 18 is a film stack consisting of: a beaded
diffuser film 22 with an outer beaded layer facing the front of the
backlight/display as shown; a prismatic brightness enhancement film
24 (Vikuiti.TM. Brightness Enhancement Film-III-Transparent
(BEFIII-T), available from 3M Company) oriented as shown, with
prisms facing the front of the backlight/display; and a reflective
polarizer 26 (Vikuiti.TM. Dual Brightness Enhancement Film-Diffuse
440 (DBEF-D440), available from 3M Company). The reflective
polarizer 26 can be considered to form the outermost film or layer
of the backlight 14.
The backlight 14 illuminates LCD panel 12, which comprises an
electronically addressable liquid crystal array sandwiched between
glass plates. The panel 12 also includes a front and back absorbing
polarizer, and a color filter matrix.
Rather than requiring a spatially non-uniform diffuser plate that
must be in registration with the light sources in order to achieve
the required illumination uniformity over the useful area of the
backlight, it would be desirable to use an alternative technique
that does not require spatial nonuniformities to provide the
bulb-hiding function.
BRIEF SUMMARY
The present application discloses, inter alia, direct-lit
backlights that include a diffuse reflector disposed in front of a
light source. The diffuse reflector has a continuous phase material
and a disperse phase material, at least one of which is
birefringent, and the disperse phase material is preferably formed
into particles having a transverse dimension less than 1 micrometer
for efficient scattering of visible light. The continuous and
disperse phases preferably have refractive indices tailored so that
the diffuse reflector transmits light from the light source
substantially more strongly for obliquely incident light than for
normally incident light. The percent transmission of the diffuse
reflector for the obliquely incident light differs from the percent
transmission of the diffuse reflector for normally incident light
by an amount .DELTA.T that preferably exceeds the combined
reflectivity due to normal-incidence surface reflections from
opposed major surfaces of the diffuse reflector. Stated
differently, the increased transmission .DELTA.T at oblique
incidence (relative to normal incidence) is greater than any
corresponding reduction in reflectivity of the opposed major
surfaces of the diffuse reflector. This angle-dependent
transmissivity gives the diffuse reflector a significant
bulb-hiding function without the need for spatial nonuniformities,
or precise registration of such nonuniformities with the light
source(s).
If one considers light propagating through the diffuse reflector at
angles that deviate more and more from normal incidence, i.e.,
light propagating at more and more oblique angles, such light has a
longer and longer optical path in traversing the thickness of the
diffuse reflector, from one major surface thereof to the opposite
major surface. For reflectors having a continuous/disperse phase
construction, this means that such light will encounter more and
more interfaces between the continuous and disperse phases, leading
to increased diffuse reflectivity and decreased transmissivity with
increasing angle. To offset this effect, the refractive indices of
the continuous and disperse phases exhibit a relative refractive
index mismatch .DELTA.nx along an x-direction and a relative
refractive index mismatch .DELTA.nz along a z-direction normal to
the x-direction, .DELTA.nz being sufficiently smaller in magnitude
than .DELTA.nx to provide the substantial bulb-hiding function. The
x-direction is parallel to the plane (e.g. even a localized
portion) of the diffuse reflector, and the z-direction is
perpendicular or normal to such plane. The refractive index
mismatch .DELTA.nz is smaller than .DELTA.nx by an amount
sufficient to overcome the effects of increasing path length with
angle, and any other effects that tend to decrease transmission
with increasing incidence angle. Preferably, light that is incident
in the x-z plane and whose polarization direction lies in the x-z
plane is strongly reflected and weakly transmitted when incident
normally on the diffuse reflector, and more weakly reflected and
more strongly transmitted at increasingly oblique angles of
incidence. Although suitable refractive index relationships will
depend on the details of construction of the diffuse reflector,
such as the overall film thickness, volume fraction of disperse
phase, particle size distribution of the disperse phase, and so on,
in some embodiments the magnitude of .DELTA.nx is at least 0.1,
more preferably 0.15 or 0.2 or more, and the magnitude of .DELTA.nz
is less than 0.03, or 0.02, or 0.01, preferably being about 0.
A light redirecting element such as a secondary diffuser or
microstructured turning film can be provided in front of the
diffuse reflector so that much of the light exiting the diffuse
reflector at high propagation angles can be forward scattered
toward directions closer to the viewing axis of the backlight.
Further, a back reflector is preferably disposed behind the light
source so that light reflected by the diffuse reflector can be
converted to a polarization and/or a direction that will be
transmitted by the diffuse reflector, thus improving backlight
efficiency by a recycling mechanism. The back reflector may reflect
light specularly or diffusely, or a combination thereof.
The angle-dependent diffuse reflector can be a diffuse polarizer,
wherein normally incident light of orthogonal polarization states
have substantially different reflection and/or transmission
properties, or a diffuse mirror, wherein such light does not have
substantially different reflection and/or transmission properties.
If a polarizer, then a refractive index mismatch .DELTA.ny between
the continuous and disperse phases along an in-plane y-direction
perpendicular to the x- and z-directions can be small relative to
.DELTA.nx, e.g. about equal to .DELTA.nz, such that light of any
incident direction whose polarization direction is parallel to the
y-z plane is weakly reflected and strongly transmitted. In such
case the backlight may include a second polarizer, preferably a
reflective polarizer, disposed proximate the angle-dependent
diffuse polarizer and oriented to reflect or otherwise block the
polarization state strongly transmitted by the angle-dependent
diffuse polarizer. I.e., the second polarizer and the
angle-dependent diffuse polarizer have respective (normal
incidence) pass axes and block axes, and the block axis of the
second polarizer is substantially aligned with the pass axis (in
this case the y-axis) of the angle-dependent diffuse polarizer.
This relative orientation preferably blocks light polarized along
the pass axis of the diffuse reflective polarizer, preventing such
light from directly exiting the backlight. Such light can be
reflected by the second polarizer back towards the light source and
at least partially converted to an orthogonal polarization state by
the back reflector. The orthogonally polarized light may then pass
through the second polarizer, and may also pass through the
angle-dependent diffuse reflective polarizer depending on the
incidence angle of such light on the diffuse reflective
polarizer.
The diffuse reflective polarizer desirably exhibits a relatively
low transmission, e.g., less than 50%, or 25%, or 10% or less, of
normally incident light whose linear polarization state is parallel
to the block axis of the diffuse reflective polarizer. However, as
the incidence angle increases from normal incidence, and provided
the incident direction lies in a plane that contains the block axis
and the original normal direction, the transmissivity of the
diffuse reflective polarizer increases for linearly polarized light
whose electric field vector lies in the plane of incidence,
referred to as p-polarized light. Thus, the diffuse reflective
polarizer possesses a polarization-sensitive bulb-hiding property,
where light of a particular polarization state is predominantly
reflected or otherwise scattered at near-normal incidence angles
(e.g. when viewed directly above the light source) but increasingly
transmitted at increasing incidence angles. This is accomplished by
appropriate selection of the relative refractive indices of the
continuous and disperse phase materials used in the diffuse
reflective polarizer as discussed above.
If the angle-dependent diffuse reflector is more representative of
a mirror, then the refractive index mismatch .DELTA.ny between the
continuous and disperse phases along the in-plane y-direction can
be on the order of .DELTA.nx, or even greater. Then, reflection and
transmission characteristics in the y-z plane may be the same as or
similar to such characteristics in the x-z plane. Light that is
incident in the y-z plane and whose polarization direction lies in
the y-z plane is strongly reflected and weakly transmitted when
incident normally on the diffuse reflector, and more weakly
reflected and more strongly transmitted at increasingly oblique
angles of incidence.
Like the angle-dependent diffuse reflective polarizer discussed
above, the diffuse reflective mirror possesses a
polarization-sensitive bulb-hiding property, but for the mirror
light of any polarization state is predominantly reflected or
otherwise scattered at near-normal incidence angles (e.g. when
viewed directly above the light source). At higher incidence
angles, the diffuse reflective mirror increasingly transmits
p-polarized light, where p-polarized light is understood as
linearly polarized light whose electric field vector oscillates in
the plane of incidence. In contrast, the orthogonal s-polarized
light is generally reflected or scattered more as the incidence
angle increases. Using a suitable back reflector, such reflected
light can be at least partially converted to high-incident angle
p-polarized light which can then be transmitted by the diffuse
reflective mirror for enhanced system efficiency. The angle- and
polarization-dependent transmission properties of the diffuse
mirror are accomplished by appropriate selection of the relative
refractive indices of the continuous and disperse phase materials,
as discussed above.
Curiously, the angle-dependent reflectivity of p-polarized light
for the disclosed diffuse reflectors (whether polarizer or mirror)
are significantly different than their multilayered counterparts
e.g. as disclosed in U.S. Pat. No. 5,882,774 (Jonza et al.), the
latter of which have the same refractive index relationships but
with a flat, extended multilayer geometry rather than a
continuous/disperse phase geometry.
These and other aspects of the present application will be apparent
from the detailed description below. In no event, however, should
the above summaries be construed as limitations on the claimed
subject matter, which subject matter is defined solely by the
attached claims, as may be amended during prosecution.
BRIEF DESCRIPTION OF THE DRAWINGS
Throughout the specification, reference is made to the appended
drawings, where like reference numerals designate like elements,
and wherein:
FIG. 1 is a schematic sectional exploded view of the optical
components of a commercially available LCD-TV, which includes a
direct-lit backlight having a diffusely reflective back
reflector;
FIG. 2 is a schematic side view of a direct-lit backlight including
a diffuse reflector having a continuous/disperse phase
construction;
FIG. 2a is a schematic top or front view of the backlight of FIG. 2
in which the diffuse reflector is a polarizer, the view of FIG. 2a
showing the relative orientation of the diffuse reflective
polarizer and a second polarizer;
FIG. 3 is a schematic perspective view of a particle of disperse
phase material embedded in a continuous phase material; and
FIG. 4 is a schematic side view of a direct-lit display that
includes a continuous/disperse phase diffuse reflector as disclosed
herein.
DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
FIG. 2 shows a schematic side view of a portion of a direct-lit
backlight 30. The backlight includes a light source 32 and a
diffuse reflector 34 that has unique angle-dependent reflectivity
properties, and an optional polarizer 38. The backlight is shown in
relation to a Cartesian x-y-z coordinate system, where the z-axis
is parallel to a viewing axis of the backlight, and the output face
or area of the backlight extends along the x- and y-directions.
The light source can be an extended diffuse source such as a cold
cathode fluorescent lamp (CCFL), or a smaller area solid state
source such as an LED. The light source 32 may also be one of an
array of such sources. If it is an array of LEDs, each LED may emit
white light, or groups of LEDs may emit different visible colors,
which when combined temporally and spatially produce the appearance
of white light. The LED(s) may be forward emitters or side
emitters, or a combination thereof. To some extent, however, the
source 32 typically emits light both in a forward direction, i.e.,
along the viewing axis of the backlight, and in sideways or oblique
directions, as shown. The angle .theta., measured from the viewing
axis of the backlight or from the normal axis of the reflector 34,
provides a measure of the direction of the light ray. In FIG. 2 the
propagation or incidence angle .theta. is measured in air. After
the light has entered the diffuse reflector 34, the propagation
direction of light within the diffuse reflector can be
characterized by an internal angle of propagation .theta..sub.int,
where .theta..sub.int is also measured with respect to the viewing
axis of the backlight or the normal axis of the reflector. In the
case of FIG. 2, .theta..sub.int is related to .theta. by the laws
of refraction at a first major surface 34a of the reflector 34.
The diffuse reflector 34 has opposed major surfaces 34a, 34b and a
physical thickness therebetween measured along the z-axis. The
major surfaces may both be exposed to air, or one or both may be
exposed to another material such as by lamination to a plate or
film. The reflector 34 provides reflectivity as a result of a
continuous/disperse phase construction, where microscopic particles
34c of a disperse phase material are dispersed or embedded in a
continuous or substantially continuous phase material 34d. In
exemplary embodiments, the disperse phase material and the
continuous phase material are both polymers, but other materials
may also be used. Reference is made to U.S. Pat. No. 6,760,157
(Allen et al.). For optimal scattering properties, the particles of
the disperse phase are on the order of the size of the wavelength
of light of interest, or smaller. For example, since backlights are
normally intended for human viewing at visible wavelengths, the
particles preferably have (on average) a transverse dimension less
than 1 micrometer, more preferably less than 0.5 or 0.1
micrometer.
The diffuse reflector 34 of FIG. 2 is a diffuse reflective
polarizer. Thus, the continuous and disperse phase materials have
been selected, and the film has been processed in such a way (e.g.
by an unconstrained uniaxial stretch at an appropriate temperature)
that the refractive index mismatch .DELTA.nx between the continuous
and disperse phases along the x-axis is large, while the refractive
index mismatch .DELTA.ny between the continuous and disperse phases
along the y-axis is small or zero. Significantly, the refractive
index mismatch .DELTA.nz between the continuous and disperse phases
along the z-axis is also controlled to be zero or small relative to
.DELTA.nx as described above.
In order to demonstrate the functionality of the backlight, we see
in the figure light rays 35a, 36a incident normally on the diffuse
reflector (or reflective polarizer) 34. Light ray 35a is linearly
polarized in the x-z plane, and light ray 36a is linearly polarized
in the y-z plane. Ray 36a is substantially transmitted by the
reflector 34, since it encounters virtually no refractive index
difference between the continuous and disperse phases. Note that
the relatively small reflections at the air/polymer interfaces
corresponding to the two major surfaces 34a, 34b, are not shown in
the figure for simplicity. Ray 35a is substantially reflected by
the reflector 34, since it encounters a relatively large refractive
index difference between the continuous and disperse phases, and
the concentration of disperse phase particles (or volume percent of
disperse phase material) and the thickness of the reflector 34 are
suitably selected to provide substantial reflection. Thus, even if
light source 32 emits unpolarized light, a substantial portion of
such light that is directed along the viewing axis of the backlight
is reflected or blocked by diffuse reflector 34, providing at least
some bulb-hiding. Further bulb hiding can be achieved by placing
another polarizer 38, preferably a low loss reflective polarizer
such as those described in U.S. Pat. No. 5,882,774 (Jonza et al.),
atop or otherwise in front of the diffuse reflector 34, making sure
to orient the second polarizer 38 substantially orthogonally with
respect to the diffuse reflector 34 such that the polarization
state that is best transmitted by the diffuse reflector 34 at
normal incidence is substantially reflected by the second polarizer
38. This is best shown in the exploded view of FIG. 2a, where
polarizer 38 has a normal incidence pass axis 38p and diffuse
reflector 34 has a normal incidence pass axis 34p, and polarizer 38
is rotated or oriented such that pass axis 38p is substantially
orthogonal to pass axis 34p (and thus a block axis of polarizer 38
is substantially aligned with pass axis 34p). The combination of
the diffuse reflector 34 and second polarizer 38 can block a large
portion of forward propagating light from the light source 32 for
very effective bulb hiding. Note that the second polarizer 38 may
also be positioned behind the diffuse reflector 34, between the
diffuse reflector and the light source 32. Regardless of the
position of the second polarizer, both it and the diffuse reflector
34 preferably extend in both length and width (the x- and
y-directions) to substantially fill the output area of the
backlight. Thus, the second polarizer and the diffuse reflector 34
are preferably substantially co-extensive in length and width.
Further, the diffuse reflector 34 and the second polarizer 38 can
be laminated together, whether directly or through one or more
intermediate films or substrates, for ease of handling.
FIG. 2 also shows an obliquely propagating light ray 37a, incident
on the major surface 34a of diffuse reflector 34 at angle .theta..
Ray 37a is polarized in the x-z plane, similar to ray 35a, but
since the x-z plane is also the plane of incidence for ray 37a, ray
37a is p-polarized with respect to the diffuse reflector 34. This
p-polarized light has a component of its electric field vector
parallel to the z-axis, and this component increases with
increasing angles .theta. and .theta..sub.int. We have found that
it is possible to process the diffuse reflector in such a way that
a substantial increase in transmission of p-polarized light can be
observed for the obliquely incident light relative to normally
incident light of the "same" polarization state (i.e., linearly
polarized in the same plane, since normally incident light is
typically not considered to be either s- or p-polarized). In this
regard, "substantial increase in transmission" refers to a
difference in transmission .DELTA.T, between the oblique ray and
the normal ray of the same polarization state, that is greater than
the reflectivity at normal incidence due to the surface reflections
at both major surfaces 34a, 34b. Stated differently, "substantial
increase in transmission" refers to a difference in transmission
.DELTA.T that cannot be attributed to the reduction or absence of
surface reflections at the major surfaces 34a, 34b for the oblique
ray. Such reduction or absence of surface reflections may, for
example, be due to the fact that the oblique ray is incident at an
angle .theta. equal to or close to the Brewster angle of the
diffuse reflector 34. The processing conditions for achieving this
increase in transmission are believed to produce a very small
refractive index difference .DELTA.nz between the continuous and
disperse phases. Thus, as the p-polarized ray propagates more and
more obliquely through the body of the diffuse reflector, it
encounters less and less of an index difference between the
continuous phase and disperse phase, and this effect overcomes
competing effects due to increased path length. Preferably, the
increase in transmission is at least 10% or at least 20% for an
angle .theta. (in air) of 60 degrees, relative to normal incidence
(.theta.=0).
Interestingly, the reader will recall that U.S. Pat. No. 5,882,774
(Jonza et al.) teaches, among other things, that controlling the
out-of-plane refractive index difference (.DELTA.nz) between layers
in a multilayer stack to be zero produces an interfacial
reflectivity that is constant with increasing incidence angle. This
can be contrasted to a film reflectivity that decreases with
increasing incidence angle as we have demonstrated with
continuous/disperse phase media.
Returning now to FIG. 2, the oblique ray 37a is seen to be more
strongly transmitted by the diffuse reflector 34. This is so even
though this ray is polarized in a plane that is parallel to the
x-axis, which would normally be considered the block axis of the
reflector 34. After exiting the reflector 34, the ray encounters
the second polarizer 38, but because of the relative orientation
shown in FIG. 2a, the ray's polarization aligns with the pass axis
of the second polarizer and proceeds out of the backlight in the
general direction of the viewer.
The reader will understand that an oblique ray similar to ray 37a
but having a polarization state like that of ray 36a, where the
electric field vector of the light oscillates parallel to the
y-axis, will be substantially transmitted by diffuse reflector 34
and substantially reflected or otherwise blocked by second
polarizer 38.
The bulb hiding capability of the angle-dependent diffuse reflector
is related to the fact that it provides substantially increased
transmission of light for obliquely propagating light rays compared
to normally incident light rays. Advantageously, this
characteristic of the diffuse reflector is not dependent on any
(macroscopic) spatial non-uniformity which would require
registering a particular portion of the diffuse reflector with the
light source(s). Although the concentration of disperse phase
material may be tailored to be higher in some macroscopic areas or
zones than others, preferably the disperse phase material is
dispersed substantially uniformly (in the form of microscopic
particles embedded in continuous phase material) throughout the
diffuse reflector.
The bulb hiding capability is also related to the fact that the
light source(s) are preferably spaced apart from the diffuse
reflector by a gap or other distance that is large enough so that
the oblique light rays can travel laterally a sufficient amount to
avoid emerging from the diffuse reflector at a point directly above
or directly adjacent the light source from which it is emitted. For
example, a gap of at least about the minimum lateral dimension of
the light source may be used. In the case of a CCFL source, the
minimum lateral dimension refers to the width of the lamp, not the
length.
The backlight 30 can include additional optical components for
improved operation. For example, a conventional diffuser film can
be placed atop or in front of the second polarizer 38 (between
polarizer 38 and observer 13) so that the predominantly obliquely
propagating light exiting the second polarizer 38 is at least
partially redirected towards the viewing axis of the backlight.
Alternatively or additionally, a brightness enhancement film (BEF)
such as any of those sold by 3M Company under the Vikuiti.TM.
brand, or conventional prismatic turning films, for example, those
sold by Mitsubishi Rayon Company, may be used to redirect obliquely
propagating light rays towards the viewing axis. In this regard,
the angle-dependent diffuse reflector can be combined with BEF or
turning film to increase the brightness, also referred to as the
"gain", of the backlight or display. Also, a back reflector can be
placed below or behind the light source 32, so that light reflected
by reflector 34 or polarizer 38 can be redirected by the back
reflector in a direction and with a polarization that can exit the
backlight, thus enhancing overall backlight efficiency and
brightness. The back reflector may be specularly reflective or
diffusely reflective. It may also be desirable to make use of
internal propagation angles .theta..sub.int in the diffuse
reflector that are super-critical, i.e., that are more oblique than
could be obtained by directing grazing incidence light (.theta.=90
degrees) in an air medium at flat major surfaces of the diffuse
reflector. Such super-critical propagation angles can be obtained
by making one or both major surfaces non-flat, e.g. by imposing a
faceted structure on them or by laminating a faceted film to one or
both flat major surfaces. Exemplary faceted films include but are
not limited to Vikuiti.TM. brand BEF products available from 3M
Company, and prismatic cube corner films. If the light source 32 is
elongated such as a conventional CCFL source or a row of LED
sources, a linear prismatic film can be used, and the axes of the
prisms can be arranged to be substantially parallel to the major
axis of the elongated source.
FIG. 3 is a schematic perspective view of a small portion of a
diffuse reflector having the unique angle-dependent properties,
showing a single particle 40 of disperse phase material embedded in
a continuous phase material 42. In the case of an exemplary
polarizer, the particle 40 may have an isotropic refractive index
of, for example, 1.57, and the continuous phase material 42 may
have refractive indices nx=1.85, ny=1.57, and nz=1.57. Such indices
are achievable using SA-115, a copolymer of polycarbonate and PCTG
available from Eastman Chemicals, for the discontinuous phase
material, polyethylene naphthalate (PEN) for the continuous phase
material, and orienting the cast film along the x-axis in an
unconstrained manner for a true uniaxial orientation along the
x-axis. In the case of an exemplary mirror, the particle 40 may
have an isotropic refractive index of, for example, 1.49, and the
continuous phase material 42 may have refractive indices nx=1.75,
ny=1.75, and nz=1.49. Such indices are achievable using
polymethylmethacrylate (PMMA) for the discontinuous phase material,
PEN for the continuous phase material, and orienting the cast film
along the x- and y-axes substantially equally under suitable
conditions. Other suitable materials for the continuous phase are
polyethylene terephthalate (PET), and copolymers of PEN and PET.
PEN and PET can be biaxially oriented such that the out-of-plane
refractive index, nz, is approximately 1.49. PMMA and other
acrylate materials can then be used to provide a match to nz of the
either material. The coPENs have slightly higher values of nz, in
the range of 1.50 to 1.54, which can be matched with various
copolyesters or inorganic materials such as glass beads. In
general, whether the diffuse reflector is more characteristic of a
polarizer or a mirror, it can be made with any suitable materials
and processed to yield refractive indices that produce the
desirable bulb-hiding properties discussed above. In many cases it
is desirable for the disperse phase material to maintain an
isotropic refractive index and the continuous phase material to be
birefringent, whether uniaxially birefringent (e.g. nx=ny.noteq.nz)
or biaxially birefringent (where no two of nx, ny, nz are the
same), but counterexamples are also possible. At least one of the
continuous phase and disperse phase material should be
birefringent. If both materials are birefringent then preferably
one is a positive and the other is a negative birefringent
material. The materials can be chosen and processed so that
nz.sub.1-nz.sub.2 is small in magnitude compared to an in-plane
refractive index differential.
The shape of the particles of the disperse phase material is not
critical, as long as the average transverse dimension is on the
order of or small compared to the wavelength of light of interest,
as discussed above. In some cases the particles may be
approximately spherical, or they may be elongated, typically along
one or two in-plane directions. For example, if the diffuse
reflector has been uniaxially stretched to orient the materials to
form a polarizer, the particles may be cigar-shaped and elongated
parallel to the stretch direction. The disperse phase material may
also be in the form of fibers aligned parallel to each other and
embedded in the continuous phase material, where each fiber may
span the entire length or width of the disperse reflector. Such
fibers may be isotropic or birefringent. If birefringent, they are
typically uniaxially birefringent, with one refractive index
associated with light polarized along the length of the fiber and a
different refractive index associated with light polarized along
any transverse direction of the fiber. In cases where the diffuse
reflector has been biaxially stretched to orient the materials to
form a mirror, the particles may be saucer-shaped, with the saucers
being oriented parallel to the plane of the mirror.
Turning our attention now to FIG. 4, we see there a schematic side
view of a direct-lit display 50 that includes a continuous/disperse
phase diffuse reflector having the desirable angle-dependent
reflection and transmission properties. The display 50 is composed
generally of a liquid crystal display panel 52, and a direct-lit
backlight 54. The display panel includes liquid crystal cells that
form picture elements (pixels) for the display, the liquid crystal
material being sealed between two glass window members or panels.
The display panel may also include a front absorbing polarizer and
a rear polarizer, not shown.
The backlight can include two or more light sources 32 as described
above. Beneath or behind the light sources is a back reflector 56,
also as described above.
In front of or above the light sources is a diffuse reflector 58
having the desirable angle-dependent properties. In the case of
display 50, reflector 58 is chosen to be a mirror-type reflector.
Thus, light from the light source 32 that strikes the reflector 58
at normal incidence is substantially reflected, regardless of its
polarization state. The reflector 58 has first and second major
surfaces 58a, 58b, between which microscopic particles 58c of a
disperse phase material are dispersed or embedded in a continuous
or substantially continuous phase material 58d. The difference in
refractive index between the continuous and disperse phase material
is large in the in-plane (x- and y-) directions, and small or zero
in the out-of-plane (z-) direction. Thus, light rays traveling at
highly oblique angles .theta..sub.int within the body of the
diffuse reflector, and which are polarized in the plane of
incidence (p-polarized), are more strongly transmitted by the
diffuse reflector. A structured surface film 60 is shown attached
to the first major surface 58a of the reflector 58. The structured
surface of film 60 may define linear extending prisms as is common
in BEF products, or other prismatic structures. The tilted or
angled nature of the facets allow obliquely propagating light rays
launched in air to propagate through the body of the reflector 58
at super-critical angles. A conventional secondary diffuser 62 is
shown attached to the second major surface 58b of the reflector.
The secondary diffuser prevents super-critical propagating light
from being trapped by total internal reflection at the second major
surface 58b, and it also redirects oblique rays at least partially
towards the viewing axis of the display. Of course, it may also
redirect some light rays traveling parallel to the viewing axis of
the display away from such viewing axis, to oblique angles.
Additional light management films may also be used, depending on
the requirements of the application. Such additional film(s), which
may include polarizers (including reflective polarizing films),
BEF, turning films, diffusing films, and the like, are shown
schematically as film 64 in FIG. 4.
We now describe test results that demonstrate the angle-dependent
operation of the disclosed birefringent diffuse reflectors.
As a baseline for comparison, several samples of commercially
available Vikuiti.TM. Diffuse Reflective Polarizing Film (DRPF),
manufactured by 3M Company, were obtained. These samples all had a
continuous/disperse phase construction. The continuous phase
material was a miscible blend of 55% PEN and 45% PET, the blend
having a refractive index of about 1.61 before orientation. The
disperse phase material of the samples was syndiotactic polystyrene
(sPS), whose weight average molecular weight is about 200,000, the
material being designated by its manufacturer, Dow Chemical Co., as
sPS-200-0. Pellets of the materials for the continuous phase and
pellets of the material for the disperse phase were heated and
mixed, in a proportion of about 58 volume % of the PEN/PET blend
and 42 vol % sPS, in a twin screw extruder at about 277 degrees C.
The mixture was continuously extruded to form a cast web about 1
millimeter thick, with small particles of the sPS dispersed within
the 55/45 PEN/PET blend matrix through the thickness of the web.
The cast web was then stretched in a length orienter (along the
length of the web, sometimes referred to as the machine direction
or MD) by a factor of about 1.35, and then stretched in a tenter
(along the width of the web, sometimes referred to as the
transverse direction or TD) by a factor of about 5.0. The stretch
factors mentioned here refer to the span of a small representative
portion of the web along a given direction (whether MD or TD) after
stretching divided by the span of that same portion of the web
before stretching. Thus, the length orienter stretch factor of 1.35
represents about a 35% increase in the span of the web along the
length direction. The length orienter utilized preheat rolls heated
to about 98.degree. C., and a quartz lamp for heating the web in
the draw gap. The tenter preheated the web to about 138.degree. C.
and then stretched it at about 118.degree. C. After stretching, the
web was converted or cut into sheets of commercial quality diffuse
reflective polarizing film, the sheets having flat major surfaces
and a thickness dependent on its processing conditions. A first
sample type, referred to as "5 mil DRPF", was processed as just
described, and had a thickness of about 00.13 mm (0.005 inch). A
second sample type, referred to as "3 mil DRPF", was thinned by
increasing the speed of the casting wheel to reduce the cast web
thickness to 0.6 mm, and had a finished thickness of about 0.08 mm
(0.003 inch). A third sample type, referred to as "2 mil DRPF", was
further thinned by increasing the casting wheel speed to reduce the
cast web thickness to about 0.4 mm, and had a finished thickness of
about 0.05 mm (0.002 inch). The two thinner cast web samples were
oriented with the same conditions as the "5 mil DRPF". Three
samples from three different crossweb positions of the oriented
film were taken of the 5 mil DRPF sample type, and one sample each
of the 3 mil DRPF sample type and of the 2 mil DRPF sample type
were tested.
Each of these samples is believed to have a similar size
distribution of disperse phase material, and similar refractive
indices of the disperse phase material and continuous phase
material in the finished product. The size distribution has an
estimated geometric mean radius for the disperse phase particles of
about 70 nanometers, the probability density associated with the
distribution having half-peak or 50% points at about 45 nm and 110
nm. Particle shapes included prolate spheroids, oblate spheroids,
ellipsoids, and near-spheres. The refractive indices of the
continuous and disperse phase materials in the finished products or
samples were estimated by using a physical model of the DRPF film
and adjusting the refractive indices to provide a good match
between modeled performance and observed hemispheric transmission
data for various incidence angles and polarizations. The results
were as shown in Table 1, where the out-of-plane refractive index
difference .DELTA.nz was estimated at slightly less than half of
the maximum in-plane refractive index difference .DELTA.nx:
TABLE-US-00001 TABLE 1 Estimated refractive index for DRPF samples
Nx ny nz 55/45 coPEN 1.767 1.605 1.525 sPS 1.575 1.613 1.613
Difference .DELTA.nx = 0.192 .DELTA.ny = 0.008 .DELTA.nz =
0.088
Each sample was tested using an Oriel Brand diode array
spectrophotometer for transmission (or transmissivity, expressed as
a percentage) at normal incidence (.theta.=0 in an air medium), and
at 30 degrees incidence (.theta.=30 degrees in an air medium), and
at 60 degrees incidence (.theta.=60 degrees in an air medium). The
spectrophotometer was operated with a diffuse (lambertian) light
source and a light collimating detector. This is optically the same
as using a collimated incident light beam and an integrating sphere
detector. For normal incidence the transmission was measured for
light linearly polarized parallel to the block axis of the
polarizing film, and for the other measurements the transmission
was measured for p-polarized light incident in a plane that was
parallel to the block axis of the polarizing film. In each case,
the transmission (T) was calculated relative to an identical setup
but where the film sample was removed. Hence, the calculated
transmission values include contributions due to reflections off of
the flat major surfaces of the samples, which were in contact with
air. Also, transmission was initially measured over a range of
discrete visible wavelengths from 400 to 700 nanometers, and these
values were then averaged to provide a single transmission value
representative of the whole visible spectrum. The results are as
shown in Table 2, where .DELTA.T is the difference in transmission
between normal incidence and 60 degree oblique incidence:
TABLE-US-00002 TABLE 2 T for Sample type Sample .theta. = 0.degree.
T for .theta. = 30.degree. T for .theta. = 60.degree. .DELTA.T 5
mil DRPF 1 17.1% 18.5% 21.2% 4.1% 2 14.1% 18.5% 4.4% 3 13.3% 17.6%
4.3% 3 mil DRPF 1 17.6% 18.9% 20.7% 3.1% 2 mil DRPF 1 18.3% 18.8%
21.5% 3.2%
Inspection of Table 2 reveals that although the standard DRPF
samples exhibit an increase in transmission .DELTA.T for
p-polarized light between 60 degrees and normal incidence, the
increase is very small, and is less than the normal-incidence
reflectivity associated with the front major surface of the sample.
Hence, the standard DRPF products do not exhibit the desired
angle-dependent behavior effective for bulb hiding purposes in a
direct-lit backlight.
Some other diffuse polarizing films were then fabricated with other
materials and using different processing conditions to provide a
much smaller out-of-plane refractive index mismatch between the
continuous and disperse phase materials.
In a first sample type, PEN was used as the continuous phase
material, and polycarbonate was used as the disperse phase
material. Mixed pellets of 0.56 intrinsic viscosity (IV) PEN and
Macrolon.TM. 2458 polycarbonate (available from Bayer corporation)
with a weight ratio of 70/30 were fed into a 25 mm twin screw
extruder at a rate of 15 lbs/hr and using a screw speed of 200 rpm.
The various extruder temperature zones were ramped from 171.degree.
C. to 277.degree. C. The remainder of the melt train and extrusion
die were held at 285.degree. C. The final melt temperature before
entering the 15 cm wide die was measured at 312.degree. C. The melt
was cast onto a wheel at 2 cm/sec. The cast wheel was cooled to
60.degree. C. The cast web was later stretched in a Karo IV brand
film stretcher from Bruckner corporation. The cast web was
preheated at 155.degree. C. for 50 sec and then stretched at that
temperature at a rate of 100%/sec. The stretch ratio was 6:1 and
the film was allowed to relax in the non-stretch direction in order
to achieve a true uniaxial stretch in portions sufficiently distant
from the clips. The finished film was a diffuse reflector
approximately 0.25 mm thick with flat major surfaces. The average
particle size was about 400 nm for the small diameter of elongated
particles within the PEN matrix. The finished film was also
substantially spatially uniform over the useable area of the film.
The refractive indices of the continuous and disperse phase
materials in the finished film were estimated to be as shown in
Table 3, based on the values of each individual material measured
when stretched as a monolithic film under the same conditions.
Since this is only an estimate, the values in Table 3 are listed
with only 2 significant digits. For this reason, .DELTA.ny and
.DELTA.nz may be as large as 0.01, even though they are listed as
0.0.
TABLE-US-00003 TABLE 3 Estimated refractive index (at 633 nm) Nx ny
nz PEN 1.82 1.58 1.58 polycarbonate 1.58 1.58 1.58 Difference
.DELTA.nx = 0.24 .DELTA.ny = 0.00 .DELTA.nz = 0.00
Two pieces of this PEN/polycarbonate sample type were tested for
transmission in the same way as the DRPF samples mentioned above.
The results are given in Table 4:
TABLE-US-00004 TABLE 4 T for T for Sample type Sample .theta. =
0.degree. .theta. = 30.degree. T for .theta. = 60.degree. .DELTA.T
PEN/polycarb 1 8.1% 12.9% 22.0% 13.9% 2 13.3% 18.8% 29.5% 16.2%
Inspection of Table 4 reveals a much higher increase in
transmission .DELTA.T for p-polarized light between 60 degrees and
normal incidence compared to the .DELTA.T measured for the DRPF
samples. The .DELTA.T for the new samples was much greater than the
normal-incidence reflectivity associated with the front major
surface of the samples. Hence, the new samples are diffuse
reflectors that do exhibit the desired angle-dependent behavior
effective for bulb hiding purposes in a direct-lit backlight.
In a second sample type, a 90/10 copolymer (coPEN) of PEN and PET
was used as the continuous phase material, and Makroblend.TM.
DP4-1386 from Bayer (a 50/50 blend of polycarbonate and PET) was
used as the disperse phase material. Mixed pellets of these
materials with a weight ratio of 50/50 were fed into a 25 mm twin
screw extruder at a rate of 15 lbs/hr and using a screw speed of
200 rpm. The various extruder temperature zones were ramped from
171.degree. C. to 277.degree. C. The remainder of the melt train
and extrusion die were held at 285.degree. C. The final melt
temperature before entering the 15 cm wide die was measured at
307.degree. C. The melt was cast onto a wheel at 2.8 cm/sec. The
cast wheel was cooled to 60.degree. C. The cast web was later
stretched in a Karo IV brand film stretcher from Bruckner
corporation. The cast web was preheated at 145.degree. C. for 50
sec and then stretched at that temperature at a rate of 100%/sec.
The stretch ratio was 6:1 and the film was allowed to relax in the
non-stretch direction in order to achieve a true uniaxial stretch
in portions sufficiently distant from the clips. The finished film
was a diffuse reflector approximately 0.18 mm thick with flat major
surfaces. The finished film was also substantially spatially
uniform over the useable area of the film. The refractive indices
of the continuous and disperse phase materials in the finished film
were estimated to be as shown in Table 5, based on the values of
each individual material measured when stretched as a monolithic
film under the same conditions. Since this is only an estimate, the
values in Table 5 are listed with only 2 significant digits. For
this reason, .DELTA.ny and .DELTA.nz may differ as much as 0.01
from the values that are listed.
TABLE-US-00005 TABLE 5 Estimated refractive index (at 633 nm) Nx Ny
nz 90/10 coPEN 1.82 1.57 1.57 polycarbonate 1.58 1.58 1.58
Difference .DELTA.nx = 0.24 .DELTA.ny = 0.01 .DELTA.nz = 0.01
One piece of this 90/10 coPEN/polycarbonate sample type was tested
for transmission in the same way as the other samples mentioned
above. The results are given in Table 6:
TABLE-US-00006 TABLE 6 T for T for T for Sample type Sample .theta.
= 0.degree. .theta. = 30.degree. .theta. = 60.degree. .DELTA.T
90/10coPEN/polycarb 1 19.0% 23.6% 32.6% 13.7%
Inspection of Table 6 again reveals a substantially higher increase
in transmission .DELTA.T for p-polarized light between 60 degrees
and normal incidence compared to the .DELTA.T measured for the DRPF
samples. The .DELTA.T for the new sample was much greater than the
normal-incidence reflectivity associated with the front major
surface of the samples. Hence, the new sample is a diffuse
reflector that does exhibit the desired angle-dependent behavior
effective for bulb hiding purposes in a direct-lit backlight.
Unless otherwise indicated, all numbers expressing quantities,
measurement of properties and so forth used in the specification
and claims are to be understood as being modified in all instances
by the term "about." Accordingly, unless indicated to the contrary,
the numerical parameters set forth in the specification and claims
are approximations that can vary depending upon the desired
properties sought to be obtained by those skilled in the art
utilizing the teachings of the present invention. At the very
least, and not as an attempt to limit the application of the
doctrine of equivalents to the scope of the claims, each numerical
parameter should at least be construed in light of the number of
reported significant digits and by applying ordinary rounding
techniques. Notwithstanding that the numerical ranges and
parameters setting forth the broad scope of the invention are
approximations, the numerical values set forth in the specific
examples are reported as precisely as possible. Any numerical
value, however, inherently contains certain errors necessarily
resulting from the standard deviations found in their respective
testing measurements.
The foregoing description is illustrative and is not intended to
limit the scope of the invention. Variations and modifications of
the embodiments disclosed herein are possible, and practical
alternatives to and equivalents of the various elements of the
embodiments would be understood to those of ordinary skill in the
art upon study of this patent document. These and other variations
and modifications of the embodiments disclosed herein may be made
without departing from the scope and spirit of the invention. All
patents and patent applications referred to herein are incorporated
by reference in their entireties, except to the extent they are
contradictory to the foregoing specification.
* * * * *